It is known that the startup of a polymer electrolyte, proton exchange membrane (PEM) fuel cell at temperatures below the freezing point of water may result in delays in startup as well as loss of performance of the fuel cell (the voltage at any current density). Causes of the performance loss include water frozen on the surface of the membrane electrode assembly and in the gas diffusion layer which impedes access of the reactant gases to the catalytic surface of the membrane electrode assembly. Another cause is freezing of water generated by the electrochemical reaction (product water) at the cathode, restricting or totally blocking the oxidant gas pathways, which significantly slows down the reaction. Ice in the fuel cell interfaces may result in high cell electrical resistance which reduces current flow and power output, slowing the process of warming the fuel cell stack.
All of these performance factors relate to the amount and location of water in the fuel cells. It is known to have shut down procedures which include draining processes to reduce the amount of water in the fuel cells, which however leaves some water within the membrane electrode assembly, as well as in coolant channels (if such are used) within the reactant gas channel plates.
Copending PCT patent application Ser. No. 04/17997, filed Jun. 2, 2004, teaches that a temperature gradient moves water through the fuel cells from one place with a higher temperature to another place with a lower temperature. Successful startup at subfreezing temperatures requires that the amount of water on the cathode side of the fuel cells be reduced to facilitate access of the oxidant, such as air, to the catalytic surface, and to increase pore volume available for newly generated product water.
Similar problems result simply from allowing an inoperative fuel cell to undergo freeze/thaw cycles.